The growing use of multiple-carrier, complex (multi-level, multi-phase), spectrally efficient waveforms such as quadrature-amplitude-modulation (QAM) and code division multiple access (CDMA) in communication systems is placing a growing demand on the power efficiency and linearity of microwave power amplifiers. Continuing reliance on vacuum electronics amplifiers such as traveling-wave tube amplifiers (TWTA) in space-based transponders and ground terminals requires tube designers to constantly develop new. techniques to improve the efficiency and linearity of the device. TWTA linearity can be greatly improved by operating the amplifiers with large output power backoff (O.B.O.) from saturation. However, this improvement in linearity is achieved at the expense of the power output and efficiency of the amplifiers. One approach to alleviate the efficiency problem is to optimize the design of the depressed collector to recover the spent beam energy for backoff operation.
To satisfy performance requirements, however, the increasing trend is for power amplifier designers as well as system designers to incorporate pre- or post-linearization modules with the power amplifiers (solid-state or TWTA) to improve overall system efficiency and linearity. There are three basic forms of linearization techniques: feedback, feedforward and predistortion. Feedback has been widely used in audio amplifiers but is of very limited use in microwave frequencies because of its bandwidth limitation due to electrical delays and possible instability problems. Feedforward is a very effective linearization technique for applications requiring both broad bandwidth and high linearity. It is relatively more complex and its effectiveness begins to deteriorate for applications with very high output power and low O.B.O. from saturation. This is primarily due to the increasing power demand on the error amplifier. Signal predistortion is a very simple and efficient linearization technique that has been used successfully for both solid-state power amplifiers (SSPA) and TWTAs. It involves cascading a predistorter in front of the power amplifier. The nonlinear characteristic of the predistorter is complementary to the distortion characteristic of the power amplifiers so that the combined system has little or no distortion. Predistortion can be implemented at the operating microwave frequency, at a convenient intermediate frequency, or at the baseband frequency. The latter option has become an increasingly attractive alternative, in part driven by the advent of high-speed digital-signal-processing circuits and digital-to-analog converters.
By far, the analog predistortion linearizer utilizing a third-order nonlinearity is the most commonly used type of predistortion implementation. The nonlinearity is usually realized with two anti-parallel diodes, a FET channel, or a low-power solid-state amplifier driven into compression, in the manner described in S.C. Cripps, Advanced Techniques in RFPower Amplifier Design, Norwood, MA: Artech House, 2002, and in R.D. Tupynamba and E. Camargo, “MESFET Nonlinearties Applied to Predistortion Linearizer Design,” IEEE MTT-S Digest, pp. 955-958, 1992, and incorporated herein by reference. Third-order linearizers are reasonably effective at suppressing nonlinear distortion for low- to medium-levels of input drive power. For drive levels close to saturation, however, a higher-order nonlinearity is necessary for effective linearization. Predistortion linearizers that utilize “curve-fitting” of the amplifier distortion characteristic during circuit design have been used to compensate higher-order distortion but such an approach is inherently dependent on the particular amplifier for which the linearizer is designed. Predistortion linearizers with individually adjustable coefficients up to fifth-order are described in the literature but are considerably more complex than third-order linearizers and are subsequently more difficult to fabricate. Furthermore, it is prohibitively difficult to extend such a configuration much beyond a fifth-order implementation.